Joshua Wahl

Advisor: Prof. Lauren Garten

will defend a master’s thesis entitled,

Developing Nanofabrication Methods and e11 Metrology to Characterize 2D Piezoelectric Materials

On

Tuesday, February 17th at 2pm

Pettit Microelectronics Room 102a & b

Or

Virtually via MS Teams:
https://teams.microsoft.com/l/meetup-join/19%3ameeting_YjYxYTQxNDQtN2E1Mi00YTBjLTk5MmEtYzE4NmExY2FiN2U1%40thread.v2/0?context=%7b%22Tid%22%3a%22482198bb-ae7b-4b25-8b7a-6d7f32faa083%22%2c%22Oid%22%3a%22b1b431e6-0dc5-43d4-a2e5-5b68b5e67d23%22%7d

Committee
Prof. Lauren Garten – School of Materials Science and Engineering (Advisor)
Prof. Eric Vogel – School of Materials Science and Engineering
Prof. Thierry Leichle – School of Electrical and Computer Engineering
           

Abstract:
2D piezoelectric materials will enable extensive thickness scaling and reduced power consumption for resonators, nanogenerators, actuators, and sensors, once the unique nanofabrication and nano-scale characterization challenges are addressed.  SnSe presents an ideal model system for the development of 2D piezoelectrics because of its high predicted in-plane e11 piezoelectric coefficients (d11 = 150 pm/V). However, SnSe can only be piezoelectric upon scaling to the monolayer limit, requiring strict layer control with in-plane orientation. Experimental validation of the piezoelectric response of SnSe has also been slowed by difficulties in device fabrication, due to the weak out-of-plane van der Waal bonding inhibiting electrode adhesion. Advancements in device fabrication and materials characterization are critically needed to reach the full potential of 2D piezoelectric technologies.

This work describes the advancement of nanofabrication processes for 2D piezoelectric devices and the development of piezoelectric metrology for the characterization of 2D materials. SnSe thin films were directly deposited via molecular beam epitaxy (MBE) and pulse laser deposition (PLD) onto lattice matched MgO single crystals and flexible mica substrates. Photolithographic processes that had previously been developed for 3D nanofabrication were not viable for 2D devices, so adaptations were made to improve metal adhesion, avoid surface oxidation, and increase device yield. X-ray photoelectron spectroscopy (XPS) was conducted after each step in the nanofabrication process, showing a clear increase in the surface oxide layer with oxygen plasma ashing and a decrease after development. Once the process was modified using guidance from the XPS results, the next step was to improve contact adhesion. Chromium interlayer successfully adhered gold electrodes to SnSe enabling co-planar device fabrication. Due to the increase adhesion inhibiting liftoff, device yield was increased by switching to indigitated electrodes (IDE) and increasing the electrode spacing to 5 µm spacing between electrodes and 1 mm spacing between sets. Using IDEs increased the active area for dielectric and piezoelectric measurements. Once viable 2D SnSe devices were fabricated the next step was to develop the characterization methodology specifically for 2D materials. Here, a piezoelectric e11 wafer-flexure metrology was developed with contactless strain measurements. Timoshenko small plate theory model was applied to enable in-plane strain to be determined across the wafer from a single point maximum displacement measurement by laser doppler vibrometer (LDV). The system was calibrated using established 3D pieozelectric materials such as AlScN. Finally, charge measurements were extracted to determine the piezoelectric coefficients of 2D SnSe. Overall, this work addresses the nanofabrication and characterization challenges needed to take the first steps towards functional 2D piezoelectric technologies.